As always, the most detailed and complete answer from "The Black Belt in Real Estate Taxation".That's exactly how I imagined it worked. 

I just set up my solo 401k, and the LLC that will be 100% owned by the 401k, and ready to buy real estate using the LLC.My question is regarding taxation.

@Eran WithanaI recommend finding an accountant who specializes in real estate taxation, tax planning and financial planning.

@Angelo LlamasI recommend finding an accountant who specializes in real estate taxation, tax planning and financial planning.

Maybe that person didn't find the right agent...I get it when an ordinary buyer comes to a new region and I have explain and convince about the taxation, transaction process, etc. 

@Paul NovakI recommend finding an accountant who specializes in real estate taxation, tax planning and financial planning.

You’ll gain valuable experience in renovations, property management, dealing with tenants, and handling the accounting and taxation aspects of long-term rentals (LTR).You can repeat this process with another duplex or scale up to a fourplex.

Or on the cash you take out.You may be able to keep the tax at 0 even with a partial exchange. 

@Michell ChaseI recommend finding an accountant who specializes in real estate taxation, tax planning and federal employee retirement planning.

San Diego is expensive to Purchase, maintenance and cap ex is expensive, property tax at purchase is expensive in terms of dollars (average SFH prop tax at purchase is ~$1k/month in San Diego), new retail (MLS) purchases are negative cash flow when allocating properly for expenses, it is tenant friendly in terms of regulations but this is largely due to the big advantages LL have due to crazy low vacancy rates.